The remarkable structural and functional diversity of the avian bill serves as an ideal system to understand how ecological and environmental factors influence morphological diversification under physically demanding tasks. For example, in cavity-excavating birds where the bill experiences tremendous physical stresses, climate and habitat affect the mechanical properties of available nesting substrates and therefore can potentially influence the shape and performance of bills. Few studies have focused on the entire avian bill, a complex structure consisting of the rhamphotheca and bony core of the upper and lower mandibles, which may evolve as independent modules or as a single integrated unit. Here, we examined patterns and drivers of bill shape diversification in barbets using micro-computed tomography of museum study skins. We employed landmark-based geometric morphometrics and phylogenetic comparative methods to study the influence of climatic conditions, integration and phylogenetic constraints on bill shape diversification. Asian and African barbets have explored distinct maxillary shape spaces without corresponding change in mandibular shape after correcting for phylogenetic non-independence. Additionally, maxillary shape for both rhamphotheca and bony components was significantly correlated with climate variables and exhibited signatures of allometric scaling. All four bill components, however, covaried with each other and have evolved in a gradual and integrated fashion, exhibiting late-stage shape disparity. Through these findings, our study offers crucial insights into how ecological behaviors impact morphological evolution of complex, multifunctional features.

Climate influences bill shape diversification in cavity-excavating birds

ABSTRACT Organisms are subject to a host of physical forces that influence morphological evolution. Birds, for instance, use their bills as implements to perform various functions, each exerting unique physical demands and selective influences on bill morphology. For example, birds that use their bills to excavate nesting or roosting cavities must resist a range of mechanical stresses to prevent fracture. However, the contribution of bill geometry and material composition to excavation stress resistance remains poorly understood. Here, we study the biomechanical consequences of bill diversification in two clades of cavity-excavating, frugivorous birds, the paleotropical barbets. Using multilayered finite element models and beam theory, we compare the excavation performance of different maxillary geometries for two loading regimes experienced by barbet bills during cavity excavation-dorsoventral impact and torsion. We find that deeper and wider maxillae perform better for impact loads than for torsional loads, with the converse for narrower maxillae. This results in a tradeoff between impact and torsion resistance imposed by bill geometry. Analytical beam models validate this prediction, showing that this relationship holds even when maxillae are simplified to solid elliptical beams. Finally, we find that composite bill structures broadly exhibit lower stresses than homogenous structures of the same geometry, indicating a functional synergy between the keratinous rhamphotheca and bony layers of the bill. Overall, our findings demonstrate the strong link between morphological evolution, behavior, and functional performance in organisms. 

https://www.biorxiv.org/content/biorxiv/early/2022/09/26/2022.09.23.509155.full.pdf

Bill shape imposes biomechanical tradeoffs in cavity-excavating birds

Organisms are subject to a host of physical forces that influence morphological evolution. Birds, for instance, use their bills as implements to perform various functions, each exerting unique physical demands and selective influences on bill morphology. For example, birds that use their bills to excavate nesting or roosting cavities must resist a range of mechanical stresses to prevent fracture. However, the contribution of bill geometry and material composition to excavation stress resistance remains poorly understood. Here, we study the biomechanical consequences of bill diversification in two clades of cavity-excavating, frugivorous birds, the paleotropical barbets. Using multilayered finite element models and beam theory, we compare the excavation performance of different maxillary geometries for two loading regimes experienced by barbet bills during cavity excavation-dorsoventral impact and torsion. We find that deeper and wider maxillae perform better for impact loads than for torsional loads, with the converse for narrower maxillae. This results in a tradeoff between impact and torsion resistance imposed by bill geometry. Analytical beam models validate this prediction, showing that this relationship holds even when maxillae are simplified to solid elliptical beams. Finally, we find that composite bill structures broadly exhibit lower stresses than homogenous structures of the same geometry, indicating a functional synergy between the keratinous rhamphotheca and bony layers of the bill. Overall, our findings demonstrate the strong link between morphological evolution, behavior, and functional performance in organisms.

/pdf/bill-shape-imposes-biomechanical-tradeoffs-in-cavity-1idq2u2i.pdf

Fruticulture in the Amazonian Rainforest is one of the main causes of deforestation, biodiversity loss, and soil erosion. Fruticulture plays a key role in the soil traits and soil organic matter (SOM) compartments by altering the soil ecosystem. Our aim was to assess the influence of Forest–Fruticulture conversion on soil traits, and SOM fractions in Brazil’s Legal Amazon. The experiment was carried out in field conditions using four land uses as main treatments: Bixa orellana, Theobroma grandiflorum, Paullinia cupana, and the Amazon Rainforest. The soil physicochemical traits were analyzed using samples that were collected from 0–5, 5–10, and 10–20 cm soil depth by using grids (10 × 10 m) with 36 sampling points. Our results showed that the Fruticulture promoted an increase in bulk density, GMD, aggregate diameter, soil porosity, gravimetric moisture, sand, clay, carbon associated with humic acid, and, the sum of bases (K+, Ca2+, and Mg2+), while the Amazon Rainforest showed the highest values of silt, soil P content, SOC, p-SOC, m-SOC, carbon associated with fulvic acid, humine, and soil C stock. Overall, the fruticulture farming systems have negative effects on SOM compartments. The results of our study highlight the importance of considering fruticulture with endemic plant species by promoting soil fertility and soil aggregation.

/pdf/forest-fruticulture-conversion-alters-soil-traits-and-soil-qcq781ie.pdf

Forest–Fruticulture Conversion Alters Soil Traits and Soil Organic Matter Compartments

For more than three decades, major efforts in sampling and analyzing tree diversity in South America have focused almost exclusively on trees with stems of at least 10 and 2.5 cm diameter, showing highest species diversity in the wetter western and northern Amazon forests. By contrast, little attention has been paid to patterns and drivers of diversity in the largest canopy and emergent trees, which is surprising given these have dominant ecological functions. Here, we use a machine learning approach to quantify the importance of environmental factors and apply it to generate spatial predictions of the species diversity of all trees (dbh ≥ 10 cm) and for very large trees (dbh ≥ 70 cm) using data from 243 forest plots (108,450 trees and 2832 species) distributed across different forest types and biogeographic regions of the Brazilian Amazon. The diversity of large trees and of all trees was significantly associated with three environmental factors, but in contrasting ways across regions and forest types. Environmental variables associated with disturbances, for example, the lightning flash rate and wind speed, as well as the fraction of photosynthetically active radiation, tend to govern the diversity of large trees. Upland rainforests in the Guiana Shield and Roraima regions had a high diversity of large trees. By contrast, variables associated with resources tend to govern tree diversity in general. Places such as the province of Imeri and the northern portion of the province of Madeira stand out for their high diversity of species in general. Climatic and topographic stability and functional adaptation mechanisms promote ideal conditions for species diversity. Finally, we mapped general patterns of tree species diversity in the Brazilian Amazon, which differ substantially depending on size class.

Giants of the Amazon: How does environmental variation drive the diversity patterns of large trees?

Tree diversity and composition in Amazonia are known to be strongly determined by the water supplied by precipitation. Nevertheless, within the same climatic regime, water availability is modulated by local topography and soil characteristics (hereafter referred to as local hydrological conditions), varying from saturated and poorly drained to well-drained and poten-tially dry areas. While these conditions may be expected to influence species distribution, the impacts of local hydrological conditions on tree diversity and composition remain poorly understood at the whole Amazon basin scale. Using a dataset of 443 1-ha non-flooded forest plots distributed across the basin, we investigate how local hydrological conditions influence 1) tree alpha diversity, 2) the community-weighted wood density mean (CWM-wd) – a proxy for hydraulic resistance and 3) tree species composition. We find that the effect of local hydrological conditions on tree diversity depends on climate, being more evident in wetter forests, where diversity increases towards locations with well-drained soils. CWM-wd increased towards better drained soils in Southern and Western Amazonia. Tree species composition changed along local soil hydrological gradients in Central-Eastern, Western and Southern Amazonia, and those changes were correlated with changes in the mean wood density of plots. Our results suggest that local hydrological gradients filter species, influencing the diversity and composition of Amazonian forests. Overall, this study shows that the effect of local hydrological conditions is pervasive, extending over wide Amazonian regions, and reinforces the importance of accounting for local topography and hydrology to better understand the likely response and resilience of forests to increased frequency of extreme climate events and rising temperatures.

https://ore.exeter.ac.uk/repository/bitstream/10871/131442/1/Marca%e2%80%90Zevallos_Local%20hydrological%20conditions%20tree%20diversity%20composition%20Amazon.pdf

Manuel J. Marca‐Zevallos

Papers

Local hydrological conditions influence tree diversity and composition across the Amazon basin